The present invention relates to a reflecting mirror for diverting a laser beam of high energy transmitted from a laser oscillator and also relates to a laser beam welding apparatus including such reflecting mirror for welding a repair sleeve snugly inserted into a thin tube, such as a heat transfer tube in a heat-exchanger, to the tube, to thereby preventing leakage through a damaged area generated in the thin tube and internally covered by the repair sleeve.
Several decades have passed since a first nuclear reactor was placed to commercial use for power generation.
Now, as one of the most important associated installations in a nuclear power generation plant, steam generators are well known for generating steam to drive a turbine. Generally, a steam generator used in a pressurized water reactor is a multitube cylindrical heat-exchanger in which secondary water is vaporized by the heat-exchanger with primary water heated in the nuclear reactor. A general internal structure of such steam generator is shown in FIG. 28, in which a lower end portion of a vertical cylindrical shell 1" is partitioned by a horizontal tube plate 2" to define a hemispherical water chamber 3". The water chamber is further divided into two halves by a vertical partition plate not shown, and end ports of a large number of thin U-shaped heat transfer tubes 5" extend through the tube plate 2" and open respectively into the two portions of the 3". These thin heat transfer tubes 5" are supported within the cylindrical shell 1" via a plurality of support plates 6". High-temperature primary water or primary reactor coolant sent from a nuclear reactor (not shown) enters through an inlet nozzle 7" communicating with one of the portions of the water chambers 3" and flows through the heat transfer tubes 5" into the other of the portions of the water chamber 3", and then it flows back to the nuclear reactor through an outlet nozzle (not shown) which communicates with the latter portion of the water chamber 3". In passing through these thin heat transfer tubes 5", heat-exchange is effected between the primary water and secondary water which is fed into the cylindrical shell 1" through a water feed nozzle (not shown) provided on a side surface portion of the cylindrical shell 1" for generating steam to be used for driving a turbine. The secondary water transformed into high-temperature steam in the above-described manner is adapted to be fed to a steam turbine (not shown) from the upper portion of the cylindrical shell 1".
Generally, in designing installations used in nuclear systems, materials used therein are carefully selected, and in addition, quality control for the materials is also very strict. In the above-described steam generator also, during its normal operation during various periodic inspections and any related repair operations, and in the event that any damage or defect should be detected in one of the thin heat transfer tubes, the following out-of-service treatment may be effected. That is, in order to prevent the high-pressure primary water from leaking out of the damaged heat transfer tube, seal plugs are pressed into the opposite end of the tube, and thereafter seal welding is effected along the peripheries of the plugs. In this manner, the damaged tube is placed in an out-of-service state.
With such out-of-service treatment with seal plugs, however, a decrease in the heat-exchange capacity of the steam generator cannot be avoided. If the number of the plugged heat transfer tubes increases, the decrease in the capacity cannot be ignored.
Considering such disadvantages, a repair method using repair sleeves was proposed and has been used. The repair sleeve is inserted into a damaged heat transfer tube and used to patch a damaged region of the tube from inside.
According to this method, it is necessary to use fairly high-temperature brazing material when the material of the heat transfer tube is special as in a nuclear steam generator. In addition to difficulty in working, in most cases metallurgical structures of the heat transfer tube and the repair sleeve are effected by this method. Furthermore, depending upon the dimensions of the water chamber where the repair work is to be conducted, it is impossible to insert a long repair sleeve into the heat transfer tubes positioned at the periphery of the chamber.
To facilitate the operation of fixing the repair sleeve on an inner surface of the damaged tube, another method has been proposed in lieu of the brazing operation mentioned above. The improved method is to mechanically or hydraulically expand a repair sleeve provided with one or more annular protrusions on an outer surface thereof. However, there is a restriction in the application of this improved method in cases where a heat transfer tube to be repaired has already been expanded and subjected to work-hardening. In such cases, because of the necessity of making the protrusions of the repair sleeve bite into the inner circumferential surface of the heat transfer tube, the material of the repair sleeve must be sufficiently harder than the material of the heat transfer tube in addition to being expansible.
For such situations, a laser beam welding apparatus was invented which is of a compact size and is applicable to welding a repair sleeve in a heat transfer tube of very small size. Patent applications on the laser beam welding apparatus were filed in various countries. Now, the U.S. patent application has been allowed and the U.S. Pat. No. 4,839,495 has issued for this invention. In the laser beam welding apparatus, a reflecting mirror of metal is used but the cooling of it is very restricted because of the compactness needed.
In such a laser welding machine, a high power laser such as a carbon dioxide gas laser or a YAG laser, has been employed. As reflecting mirrors in the prior art apparatus, mirrors produced by forming a gold coating layer 2' having a high reflectivity on a mirror base member 1' made of copper as shown in FIG. 9, or mirrors formed by providing a nickel coating layer 3' on a mirror base member to facilitate optical polishing and then forming a gold coating layer 2' thereon as shown in FIG. 10, have been generally used.
In the heretofore known reflecting mirror as described above, when it is used under high-temperature environmental conditions, or when a laser is used under a high power condition for a long period, the temperature of the reflecting mirror would rise due to absorption of several percent of the laser beam energy.
When such reflecting mirror is used without carrying out sufficient cooling such as water cooling, the mirror temperature would rise, and mutual diffusion of metals may occur between the gold coating layer and the mirror base member or between the gold coating layer and the nickel coating layer. FIG. 11 is an illustration of the state of mutual diffusion between the gold coating layer and the nickel coating layer after the reflecting mirror shown in FIG. 10 has been used for a long time under a high power condition on the basis of results of Auger analysis along the direction of depth from the mirror surface, and it is seen from this figure that nickel can diffuse up to the mirror use under surface. If such conditions continues, a reflectivity of the reflecting mirror may be lowered, and eventually breakdown of the mirror may result. FIG. 12 shows variations in the reflectivity and the mirror temperature of the reflecting mirror in FIG. 10 with respect to laser beam irradiation time, and from this figure it will be understood that the mirror reflectivity lowers about 50 seconds after irradiation with a laser beam.
In addition, breakdown of a reflecting mirror occurs also when nickel or copper coming out to the mirror surface is oxidized, and hence the beam absorptivity is further increased. In the case where a reflecting mirror is used for a long period of time under high-temperature conditions, gold on the surface of a gold coating layer may also become oxidized, thereby reducing the reflectivity.
Briefly stated, it is difficult to use the laser beam welding apparatus using the reflecting mirrors with the above-mentioned structure in continuous operation.